prevent double stack trace 'fixes'

[Rich-Harris]

please god let this be the last one of these. closes #3371

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[benmccann]

I'm not sure we should include the __SVELTEKIT_DEV__. E.g. at my last company we would show you the stacktrace if you were logged in as an employee. We wouldn't show it to customers, but it was helpful for engineers to have access to it

[Rich-Harris]

They do have access to it, in handleError. If we wanted to expose it more broadly than that then we can, but that's a much larger conversation than this change

[benmccann]

yeah, but it's harder to get at on a production machine vs having it just show up on the screen. I don't know that we should place restrictions on what people are allowed to do

[Rich-Harris]

Sure — it's a conversation worth having, but it needs to be had separately. We currently prevent stack traces from appearing in production (at one point we didn't, but we were called out for bad security practices). This PR doesn't change that and I don't think this bugfix should get bogged down in that discussion, so let's figure it out elsewhere

[benmccann]

ah, I didn't realize we were doing that. this looked like new behavior

[Rich-Harris]

alright, whatever gremlins were in the system appear to have left us alone. think this is ready